1. Field of the Invention
The present invention relates to a wafer transfer method for transferring a number of semiconductor wafers from cassettes (carriers) to a boat for heat treatment, and more particularly, to a wafer transfer method for efficiently automatically transferring semiconductor wafers when product wafers are mixed with wafers for monitoring.
2. Description of the Related Art
Semiconductor devices, such as ICs or LSIs, are manufactured by successively subjecting wafers, cut from an ingot, to a number of processes, including surface treatment, thermal oxidation, impurity diffusion, film deposition, etching, etc. During these manufacturing processes, each semiconductor wafer is repeatedly heat-treated.
Vertically stacked multi-stage furnaces, e.g., four-stage furnaces, are generally used as furnaces for the heat treatment of semiconductor wafers. A multi-stage holder is disposed in front of an inlet of each furnace. Various automatic devices for handling the wafers are mounted on the holder.
The semiconductor devices are loaded into or unloaded from the heating furnaces by means of wafer boats for exclusive use. Usually, each boat carries a maximum of 200 wafers as a lot.
Meanwhile, the semiconductor wafers are transported from a preceding stage to the heating furnaces by means of exclusive-use cassettes (or carriers). Usually, each cassette contains a maximum of 250 wafers as a lot. Accordingly, the wafers must be transferred from a plurality of cassettes to one boat.
In handling the wafers, an operators' hands are prohibited from directly touching them, for prevention of contamination. To attain this, an exclusive-use automatic wafer transfer apparatus, for use as incidental equipment for each heating furnace, is located in the vicinity of the furnace inlet. This apparatus serves to transfer the wafers automatically from the cassettes to each boat.
In general, semiconductor device manufacturers do not inspect all of heat-treated wafers, but they use a method wherein two or three wafers for monitoring (hereinafter referred to as monitor wafers) are sampled from the boat after the end of the heat treatment process, and the sampled monitor wafers are inspected for the thickness of film on their surface and other conditions. If the inspection results on the monitor wafers are acceptable, then product wafers are regarded as acceptable. Thus, the monitor wafers are arranged in predetermined positions at intermediate portions on either end side of an array of wafers on the boat.
Conventionally, in transferring these monitor wafers to a predetermined positions on the boat, each of them is stored beforehand in each cassette so that they are automatically transferred together with the product wafers, from the cassettes to the boat, by means of the wafer transfer apparatus.
According to this conventional method, however, the monitor wafers are previously stored in the cassettes, so that the product wafers cannot be fully contained in each cassette. If one monitor wafer is stored beforehand in each cassette, for example, only 24 product wafers can be contained in the cassette. Thus, the number of handleable product wafers per cassette is limited, so that the overall process efficiency is low.
Instead of mechanically transferring the monitor wafers by means of the wafer transfer apparatus, the operators may manually transfer the monitor wafers to the boat. In this case, however, the monitor wafers will be soiled or damaged, thereby entailing a reduction in the cleanliness of room air.
Moreover, each cassette does not always contain an entire lot of 25 wafers. If the wafers in the cassette are transferred directly to the boat when they are subject to an omission, they cannot be continuously arranged at regular pitches on the boat. As a result, the wafers inevitably undergo uneven heat treatment.